1. Field of the Invention
This invention relates to a method for the production of oxydicarboxylic acid salts. More particularly, this invention relates to a method for the production of an oxydicarboxylic acid salt by the liquidphase dehydrogenation of corresponding polyethylene glycols possessing 2 to 14 oxyethylene units.
2. Description of Prior Art
Oxydicarboxylic acid salts excel in chelating ability and exhibit satisfactory biodegradability and, therefore, are accepted as useful compounds for builders in synthetic detergents. Particularly in recent years, they have come to attract keen attention because tripolyphosphates have been held responsible for eutrophication of rivers, brooks, lakes and swamps, and the existing builders for detergents have been moving toward substitution with new substances. Besides excelling in chelating ability and biodegradability, oxydicarboxylic acid salts exhibit high compatibility with surface active agents and possess outstanding properties befitting builders for detergents. This fact is directing growing interest to use such oxydicarboxylic acid salts either independently or in combination with builders such as zeolite.
As means of producing oxydicarboxylic acids, it is known to oxidize polyethylene glycols to corresponding oxydicarboxylic acids with nitric acid, and with a gas containing molecular oxygen in the presence of a catalyst of noble metal such as palladium (U.S. Pat. No. 3,929,873), and to dehydrogenate in the presence of a caustic alkali. The method which produces oxydicarboxylic acid salts by the oxidation of polyethylene glycols with nitric acid requires use of nitric acid of high concentration and, moreover, entails the problem of corrosion of the equipment used for the oxidation and suffers from the disadvantage that oxydicarboxylic acid salts produced by the oxidation are difficult of separation and purification. In the case of the method which oxidizes by the use of a noble metal such as platinum or palladium, the disadvantage that the catalyst is expensive and, consequently, the production cost is high and the difficulty of separation of aldehyde which is formed by oxidation have posed itself a serious problem on the way of commercialization of this method.
The method which produces oxydicarboxylic acid salts by liquid-phase dehydrogenation of alcohols in a caustic alkali in the absence of a catalyst has the disadvantage that since the reaction proceeds only at an elevated temperature in the neighborhood of 300.degree. C., alcohols such as polyethylene glycols which have ether bonds in the molecular units thereof under such reaction conditions by-produce glycolates, acetates, and oxalates in large amount and produce oxydicarboxylic acid salts in low yields because of cleavage of the ether bonds.
U.S. Pat. No. 2,384,817 discloses a method for the production of carboxylates by the dehydrogenation of alcohols such as, for example, ethanol, propanol, ethylene glycol, polyethylene glycol and momoethanol amine in the presence of a caustic alkali, which method permits use of a metal such as cadmium, copper, nickel, silver, lead or zinc or a compound of such a metal as the catalyst for the reaction. Further, U.S. Pat. No. 2,384,817 contains statements indicating that in the catalysts mentioned above, cadmium and compounds thereof exhibit the best effects, that copper compounds exhibit high initial activity but the duration of maximum catalytic activity is short, and that the other catalysts, i.e. nickel, silver, lead and zinc compounds exhibit notably lower levels of activity than copper and cadmium compounds.
U.S. Pat. No. 3,717,676 limits the alcohols set forth in the aforementioned U.S. Pat. No. 2,384,817 to only polyethylene glycols and lays down the requirement that the reaction should be carried out in the presence of a cadmium catalyst. The fact that this method uses cadmium as a catalyst, however, poses itself a problem on the way to commercialization of this method.
Japanese Patent Publication No. SHO 57(1982)-5,775 discloses a method which, by use of a catalyst containing copper and/or nickel, produces disodium oxydiacetate in high yields. When the reaction proceeds in the presence of nickel or a nickel-containing compound is used as catalyst, oxalates and carbonates occur in large amounts because of cleavage of ether bonds and, consequently, disodium oxydiacetate is produced in low yields. An attempt to obtain disodium oxydiacetate of high purity from the resultant reaction mixtures, therefore, necessitates a complicate process for purification. On the other hand, the catalyst formed solely of copper which exhibits high activity at an elevated reaction temperature suffers from serious loss of catalytic activity and withstands only one of two cycles of reaction. When the reaction temperature is lowered for the purpose of elongating the service life is the catalyst, the reaction velocity is lowered too much for the reaction itself to be practicable.
U.S. Pat. No. 4,110,371 discloses a method for the production of oxydicarboxylic acid salts by the reaction of corresponding polyethylene glycols at elevated temperatures in the presence of a nickel-copper-chromia catalyst. This method, however, produces oxydicarboxylic acid salts in low yields.
An object of this invention, therefore, is to provide a novel method for the production of an oxydicarboxylic acid salt.
Another object of this invention is to provide a method for the production of an oxydicarboxylic acid salt by the liquid-phase dehydrogenation of polyethylene glycols having 2 to 14 oxyethylene units.